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    <script>
        var arr = [1, [2, 3], [4], 5]// [1,2,3,4,5]
        // 思路一：
        // console.log(arr.toString().split(',').map(item => item - 0));
        // 思路二：es6的flat
        console.log(arr.flat(Infinity));
    </script>
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